Organic monolayer passivation and silicon heterojunction photovoltaic devices using the same

ABSTRACT

A method for inorganic surface passivation in a photovoltaic device includes etching a native oxide over an inorganic substrate, the inorganic substrate having a surface; and forming an organic monolayer on the surface of the inorganic substrate to form a heterojunction, the organic monolayer having the following formula: ˜X—Y, wherein X is an oxygen or a sulfur; Y is an alkyl chain, an alkenyl chain, or an alkynyl chain; and X covalently bonds to the surface of the inorganic substrate by a covalent bond.

PRIORITY

This application is a continuation of and claims priority from U.S.patent application Ser. No. 14/749,956, filed on Jun. 25, 2015, entitled“ORGANIC MONOLAYER PASSIVATION AND SILICON HETEROJUNCTION PHOTOVOLTAICDEVICES USING THE SAME,” the entire contents of which are incorporatedherein by reference.

BACKGROUND

The present invention generally relates to semiconductors, and morespecifically, to passivation processes directed to photovoltaic devices.

Organic materials are of interest for photovoltaic applications forvarious reasons. For example, organic materials are relatively low incost and can be processed over large surface areas, e.g., on flexiblelow-cost substrates. However, the efficiency of photovoltaic deviceswith such organic materials is lower than devices with inorganicmaterials. The smaller diffusion length of minority carriers in organicmaterials, compared to inorganic materials, can decrease efficiency.

Accordingly, heterojunction devices, such as in photovoltaic devices,that include an inorganic substrate (an absorption layer), for examplesilicon, with an organic contact are particularly attractive. Suchdevices combine the organic material's low-temperature, large-areaprocessing capability with the inorganic material's large diffusionlength, which substantially eliminates excessive recombination in theabsorption layer.

In heterojunction devices, the dangling bonds at the surface of theinorganic material are passivated to minimize recombination loss at theorganic/inorganic interface. Wide band gap materials, for example, PQ(9,10-phenanthrenequinone), have been used to passivate the surface ofsilicon (Si). Referring to FIG. 1, the lowest unoccupied molecularorbital (LUMO)/conduction band (E_(c)) offset in PQ repels minorityelectrons, which is favorable for reducing dark current. However,because of the highest occupied molecular orbital (HOMO)/valence band(E_(v)) offset at the Si/PQ interface, the potential barrier may reducethe collection of majority holes at the emitter junction and drasticallyreduce photocurrent. These effects hamper the use of maturehole-transport organic materials, such as pentacene (also shown in FIG.1), to form a high-performance heterojunction solar cell devices onn-type silicon.

SUMMARY

In one embodiment of the present invention, a method for inorganicsurface passivation in a photovoltaic device includes etching a nativeoxide over an inorganic substrate, the inorganic substrate having asurface; and forming an organic monolayer on the surface of theinorganic substrate to form a heterojunction, the organic monolayerhaving the following formula: ˜X—Y, wherein X is an oxygen or a sulfur;Y is an alkyl chain, an alkenyl chain, or an alkynyl chain; and Xcovalently bonds to the surface of the inorganic substrate by a covalentbond.

In another embodiment, a method for inorganic surface passivation in aphotovoltaic device includes etching a native oxide over an inorganicsubstrate, the inorganic substrate including silicon and having asurface; and forming an organic monolayer on the surface of theinorganic substrate to form a silicon heterojunction, the organicmonolayer having the following formula: ˜X—Y—Z, wherein X is an oxygenor a sulfur; Y is an alkyl chain, an alkenyl chain, or an alkynyl chain;Z is a methyl group or an epoxy group; and X covalently bonds to thesurface of the inorganic substrate by a silicon-oxygen bond or asilicon-sulfur bond.

Yet, in another embodiment, a heterojunction in a photovoltaic deviceincludes an inorganic substrate having a surface; and an organicmonolayer on the surface of the inorganic substrate, the organicmonolayer having the following formula: ˜X—(CH₂)_(n)—CH₃, wherein X isan oxygen or a sulfur; n is an integer from about 4 to about 21; and Xcovalently bonds to the surface of the inorganic substrate to form acovalently bonded ether or a covalently bonded thioether on the surface;an organic semiconductor material on a surface of the organic monolayer,the organic semiconductor material being substantially free of doping;and a conductive electrode disposed onto at least on a portion of theorganic semiconductor material.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a schematic diagram of HOMO/E_(v) and LUMO/E_(c) bandoffsets in silicon heterojunctions;

FIGS. 2-4 illustrate various embodiments of inorganic surfacepassivation according to the present invention, in which:

FIG. 2 illustrates inorganic surface passivation according to a firstembodiment;

FIG. 3 illustrates inorganic surface passivation according to a secondembodiment;

FIG. 4 illustrates inorganic surface passivation according to a thirdembodiment;

FIG. 5A illustrates an exemplary semiconducting stack comprising asurface passivated silicon substrate; and

FIG. 5B is a graph comparing current density as a function of voltage inthe stacks of FIG. 5A.

DETAILED DESCRIPTION

Disclosed herein is an organic monolayer and method for passivating thesurface of an inorganic substrate, for example, a silicon substrate foruse in photovoltaic devices. The passivation layer disclosed is a thinmonolayer. As known in the art, the tunneling probability across apotential barrier depends on the barrier height and barrier thickness.The higher the barrier and the thicker the barrier, the lower thetunneling probability. However, if either of the barrier height or thebarrier thickness approaches zero, the potential barrier approacheszero; hence, the tunneling probability approaches unity. Therefore, thepotential barrier formed at the heterojunction due to the thin monolayerin accordance with the present invention is small, irrespective of anyexisting band offsets. Therefore, hole transport at the heterojunctionis minimally affected because the hole tunneling probability across thethin potential barrier is high. It is noted that like reference numeralsrefer to like elements across different embodiments.

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

As used herein, the articles “a” and “an” preceding an element orcomponent are intended to be nonrestrictive regarding the number ofinstances (i.e. occurrences) of the element or component. Therefore, “a”or “an” should be read to include one or at least one, and the singularword form of the element or component also includes the plural unlessthe number is obviously meant to be singular.

As used herein, the terms “invention” or “present invention” arenon-limiting terms and not intended to refer to any single aspect of theparticular invention but encompass all possible aspects as described inthe specification and the claims.

As used herein, the term “about” modifying the quantity of aningredient, component, or reactant of the invention employed refers tovariation in the numerical quantity that can occur, for example, throughtypical measuring and liquid handling procedures used for makingconcentrates or solutions. Furthermore, variation can occur frominadvertent error in measuring procedures, differences in themanufacture, source, or purity of the ingredients employed to make thecompositions or carry out the methods, and the like. In one aspect, theterm “about” means within 10% of the reported numerical value. Inanother aspect, the term “about” means within 5% of the reportednumerical value. Yet, in another aspect, the term “about” means within10, 9, 8, 7, 6, 5, 4, 3, 2, or 1% of the reported numerical value.

Turning now to the Figures, FIGS. 2-4 illustrate various embodiments ofinorganic surface passivation for use in photovoltaic devices accordingaccording to the present invention. FIG. 2 illustrates inorganic surfacepassivation according to a first embodiment. An inorganic substrate 210has a native oxide layer 212 on its surface. The inorganic substrate 210may include crystalline silicon, n-type silicon, p-type silicon, or anycombination thereof. The inorganic substrate 210 may include any othersuitable inorganic material, including, but not limited to, group IVsemiconductors such as Si, Ge, SiGe; and III-V semiconductors such asGaAs. The thickness of the inorganic substrate 210 is in a range fromabout 100 nm to about 1 mm; although thinner and thicker layers may beused as well. In one example, the inorganic substrate 210 is comprisedof a solar-grade (single-crystalline or multi-crystalline) silicon waferand has a thickness in the range from about 100 to about 300 micrometers(μm). In another example, the inorganic substrate 210 is comprised of athin (1-20 μm) silicon layer transferred from a host silicon wafer andbonded onto a flexible handle substrate (such as plastic) using knownlayer transfer and bonding techniques. In a further example, theinorganic substrate 210 is comprised of a 100 nm-2 μm thick GaAs layergrown epitaxially on a Ge handle substrate. In yet another example, theinorganic substrate 210 is comprised of a 100 nm-2 μm thick GaAs layertransferred from a host substrate (such as Ge or GaAs) and bonded onto aflexible handle substrate (such as plastic) using known layer transferand bonding techniques.

When the inorganic substrate 210 includes silicon, the native oxidelayer 212 is etched 220 (removed) to form Si—H bonds 230 on the etchedinorganic surface 214. When the inorganic substrate 210 includes anotherinorganic material, etching of the native oxide layer 212 forms an[inorganic atom —H] bond on the surface 214 of the inorganic substrate210. The etching can be performed by applying a dilute solution ofhydrofluoric acid (HF) to the native oxide layer 212 of the inorganicsubstrate 210 by methods known in the art. Etching to remove the nativeoxide layer 212 can be performed by other suitable methods, such ashydrogen plasma etching.

The inorganic substrate 210 is immersed in a solution of an alcohol orthiol 240 at an elevated temperature. The alcohol or the thiol 240 hasthe following formula: HX—Y, wherein H is hydrogen, X is sulfur oroxygen, and Y is an alkyl chain, an alkenyl chain, an alkynyl chain, orany combination thereof. Y can include any number of carbons, forexample, in a range from about 4 to about 21. Y is substantially free ofbranching.

When Y is an alkenyl chain, the double bonds in the chain includeseither all cis double bonds or all trans double bonds. When Y is analkyl chain, Y can have the following formula: (CH₂)_(n)CH₃, wherein nis an integer from about 4 to about 21. The integer n can be about or inany range from about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,18, 19, 20, and 21.

The reaction of the alcohol or thiol 240 with the inorganic substrate210 is promoted by heating the inorganic substrate 210. The heating canbe performed at a temperature in a range from about 90 to about 150°Celsius (° C.).

The organic atom —H bonds 230, for example, Si—H bonds, on the surface214 of the inorganic substrate 210 react with the alcohol or the thiol240 to form an organic monolayer 250 on the surface 214 of the inorganicsubstrate 210. The organic monolayer 250 passivates the surface 214 ofthe inorganic substrate 210 by saturating the dangling bonds. Theorganic monolayer 250 has the following formula: ˜X—Y, wherein X is anoxygen or a sulfur; Y is an alkyl chain, an alkenyl chain, or an alkynylchain; and X covalently bonds to the surface 214 of the inorganicsubstrate 210 by an inorganic atom-oxygen bond, for example, asilicon-oxygen bond (Si—O˜) or a silicon-sulfur bond (Si-S˜).

The organic monolayer 250 forms a passivation layer that is relativelythin, which means the potential barrier formed at the heterojunctionbetween the organic monolayer 250 and the inorganic substrate 210 issmall (irrespective of the existing band offset). Therefore, holetransport at the heterojunction is not affected.

The organic monolayer 250 has a thickness in a range from about 0.5 nmto about 5 nm. In one aspect, the organic monolayer 250 has a thicknessin a range from about 2 to about 3 nm. In another aspect, the organicmonolayer 250 has a thickness in a range from about 1 to about 2 nm.Yet, in another aspect, the organic monolayer 250 has a thickness aboutor in any range from about 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5,and 5.0 nm.

FIG. 3 illustrates inorganic surface passivation for use in photovoltaicdevices according to a second embodiment. An inorganic substrate 210comprising silicon has a native oxide layer 212 on its surface. Thenative oxide layer 212 is etched 220 (removed) to form Si—H bonds 330 onthe etched inorganic surface 214. The etching can be performed byapplying a dilute solution of HF to the native oxide layer 212 of theinorganic substrate 210 by methods known in the art.

The inorganic substrate 210 is immersed in a solution of an alcohol orthiol 340 at an elevated temperature. The alcohol or the thiol 340 hasthe following formula: HX—Y—Z, wherein H is hydrogen; X is sulfur oroxygen, Y is an alkyl chain, an alkenyl chain, an alkynyl chain, or anycombination thereof; and Z is a methyl group or an epoxy group. The Xcovalently bonds to the surface of the inorganic substrate 210 by asilicon-oxygen bond (Si-X˜) or a silicon-sulfur bond (Si-S˜). Any of theabove descriptions for Y in the alcohol or thiol 240 having the formulaHX—Y are applicable for HX—Y—Z.

The reaction of the alcohol or thiol 340 with the inorganic substrate210 is promoted by heating the inorganic substrate 210. The Si—H bonds330 on the surface 214 of the inorganic substrate 210 react with thealcohol or the thiol 340 to form an organic monolayer 350 on the surface214 of the inorganic substrate 210. The organic monolayer 350 passivatesthe surface of the inorganic substrate 210 by saturating the danglingbonds.

The organic monolayer 350 has the following formula: ˜X—Y—Z, wherein Xis an oxygen or a sulfur; Y is an alkyl chain, an alkenyl chain, or analkynyl chain; Z is a methyl group or an epoxy group; and X covalentlybonds to the surface 214 of the inorganic substrate 210 by asilicon-oxygen bond (Si—O˜) or a silicon-sulfur bond (Si-S˜). Theorganic monolayer 350 forms a passivation layer that is relatively thinand has a thickness described above for organic monolayer 250.

FIG. 4 illustrates inorganic surface passivation for use in photovoltaicdevices according to a third embodiment. An inorganic substrate 210 hasa native oxide layer 212 on its surface. The inorganic substrate 210 caninclude, for example, silicon. The native oxide layer 212 is etched 220(removed) to form organic atom —H bonds 430, for example, Si—H bonds, onthe etched inorganic surface 214. The etching can be performed byapplying a dilute solution of HF to the native oxide layer 212 of theinorganic substrate 210 by methods known in the art.

The inorganic substrate 210 is immersed in a solution of an alcohol orthiol 440 at an elevated temperature. The alcohol or the thiol 440 hasthe following formula: HX—(CH₂)_(n)—CH₃, wherein X is an oxygen or asulfur, and n is an integer from about 4 to about 21. The integer n canbe about or in any range from about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,14, 15, 16, 17, 18, 19, 20, and 21.

The reaction of the alcohol or thiol 440 with the inorganic substrate210 is promoted by heating the inorganic substrate 210. The inorganicatom —H bonds 430, for example Si—H bonds, on the surface 214 of theinorganic substrate 210 react with the alcohol or the thiol 440 to forman organic monolayer 450 on the surface 214 of the inorganic substrate210. The organic monolayer 450 passivates the surface of the inorganicsubstrate 210 by saturating the dangling bonds.

The organic monolayer 450 has the following formula: ˜X—(CH₂)_(n)—CH₃,wherein X is an oxygen or a sulfur; n is an integer from about 4 toabout 21; and X covalently bonds to the surface of the inorganicsubstrate 210 to form a covalently bonded ether (e.g.,Si—O—(CH₂)_(n)—CH₃) or a covalently bonded thioether (e.g.,Si—S—(CH₂)_(n)—CH₃) on the surface 214 of the inorganic substrate 210.The integer n can be about or in any range from about 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21. The organicmonolayer 350 forms a passivation layer that is relatively thin and hasa thickness described above for organic monolayer 250.

In the above-described embodiments in FIGS. 2, 3, and 4, hybridheterojunctions are formed between the inorganic substrate 210 and aconductive electrode (not shown). The hybrid heterojunctions furtherinclude at least one organic semiconductor material layer together withthe organic monolayers 250, 350, and 450. The organic semiconductormaterial is over the organic monolayer, and the conductive electrode isdisposed onto at least a portion of the organic semiconductor material.

The said heterojunctions are configured to effectively behave asSchottky barriers by (i) choosing the conductive electrode to have awork-function type opposite to that of the inorganic substrate (i.e., ifthe inorganic substrate is n-type, the conductive electrode is chosen tohave a high work-function, and if the inorganic substrate is p-type, theconductive electrode is chosen to have a low work-function); (ii)choosing the organic semiconductor to be substantially free ofimpurities, have a low defect density, and be sufficiently thin toensure low minority carrier recombination within the organic materials,and to ensure the organic semiconductor is fully depleted; and (iii)choosing the organic semiconductor to suppress the transport of carriershaving the same charge type as that of the majority carriers in theinorganic substrate and/or favor the transport of carriers having theopposite charge type to that of the majority carriers in the inorganicsubstrate (i.e., if the inorganic substrate is n-type (wherein electronsare the majority carriers), the organic semiconductor is chosen to be anelectron blocking and/or a hole transport material, and if the inorganicsubstrate is p-type (wherein holes are the majority carriers), theorganic semiconductor is chosen to be a hole blocking and/or an electrontransport material). Further, the organic monolayers 250, 350, and 450ensure good surface passivation of the inorganic substrate, andtherefore, a low minority carrier recombination. Under the aboveconditions, the contribution of minority carriers to the electronictransport is low, and the electronic transport is dominated by themajority carrier transport across the said Schottky barrier.

The organic semiconductor material is substantially un-doped orsubstantially free of doping. Examples of organic semiconductormaterials which can provide electron blocking (or hole transport)functions include, but are not limited to, pentacene, rubrene,anthracene, poly(3-hexylthiophene) (P3HT); tetraceno[2,3-b]thiophene;α-sexithiophene; poly(3,3′″-didodecylquaterthiophene);poly(2,5-bis(3-decylthiophen-2-yl)thieno[3,2-b]thiophene);N,N′-Bis(3-methylphenyl)-N,N′-diphenyl-benzidine (TPD);N,N′-Bis(phenanthren-9-yl)-N,N′-bis(phenyl)-benzidine (PAPB);4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP); 1,3-Bis(N-carbazolyl)benzene(mCp); 4,4′-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine](TAPC);2,2′-Dimethyl-N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine(α-NPD);9,9-Dimethyl-N,N′-di(1-naphthyl)-N,N′-diphenyl-9H-fluorene-2,7-diamine(NPB); N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine(NPD); N,N′-Di(2-naphthyl-N,N′-diphenyl)-1,1′-biphenyl-4,4′-diamine(β-NPB); Tri-p-tolylamine;4,4′,4″-Tris[phenyl(m-tolyl)amino]triphenylamine;Tris(4-carbazoyl-9-ylphenyl)amine (TCTA); Tetra-N-phenylbenzidine (TPB);1,3-Bis(triphenylsilyl)benzene; poly-aniline;poly(3,4-ethylenedioxythiophene);poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS);poly(3,4-ethylenedioxythiophene); tetracyanoethylene;poly(thiophene-3-[2-(2-methoxyethoxy) ethoxy]-2,5-diyl);bis-poly(ethyleneglycol) (PEDOT:PEG); 7,7,8,8-Tetracyanoquinodimethane,and combinations thereof.

Examples of organic semiconductor materials which can provide holeblocking (or electron transport) functions include, but are not limitedto, bathocuproine (BCP); bathophenanthroline (BPhen);3-(Biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole(TAZ); 2-(4-Biphenylyl)-5-phenyl-1,3,4-oxadiazole (PBD);bis(8-hydroxy-2-methylquinoline)-(4-phenylphenoxy)aluminum;2,5-Bis(1-naphthyl)-1,3,4-oxadiazole (BND);2-(4-tert-Butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (Butyl-PBD);Tris-(8-hydroxyquinoline)aluminum (Alq3); hexadecafluoro copperphthalocyanine (F₁₆CuPc); naphthalene diimide derivatives; perylenediimide derivatives; C₆₀; and combinations thereof.

The conductive electrode may include a metal (blanket or grid). Theconductive electrode may include, or further include, a transparentconductive material, such as a transparent conductive oxide.Non-limiting examples of transparent conductive oxides includealuminum-doped zinc-oxide and indium-tin-oxide, a conductive polymersuch as PEDOT:PSS, a 2D material such as graphene, or metalnano-structures such as silver nano-wires. Non-limiting examples of highwork-function metals include gold, platinum, palladium, and silver.Non-limiting examples of low work-function metals include erbium,strontium, and calcium. In some embodiments, the thickness of theorganic semiconductor material is in a range from about 2 nm to about200 nm. In other embodiments, the thickness of the organic semiconductormaterial is in a range from about 3 nm to about 25 nm. In yet anotherset of embodiments, the thickness of the organic semiconductor materialis in a range from about 5 nm to about 15 nm.

EXAMPLE

FIG. 5A illustrates an exemplary semiconducting stack comprising asurface passivated silicon substrate. It should be appreciated that theillustration of the example of FIG. 5A is not shown to scale. Thesemiconducting stack included an aluminum layer 510 with a 300 μm n-typeSi substrate 520 disposed thereon. The Si substrate 520 included apassivation layer 530. For comparison, the passivation layer 530 was a 5nm thick PQ layer or an organic monolayer as disclosed herein. A 10 nmpentacene layer 540 was disposed on the passivation layer 540. A 1 mm²gold pad 550 was disposed on the pentacene layer 540. These devices wereprepared to demonstrate the differences between Si/PQ heterojunctionstructures and Si/organic monolayer heterojunction structures.

FIG. 5B is a graph comparing current density as a function of voltage inthe Si/PQ (560) and Si/organic monolayer (570) heterojunction structuresof FIG. 5A. The fill factors for each of the structures were assessed.The fill factor is the ratio of the actual maximum obtainable power tothe product of the open circuit voltage and short circuit current, whichis a parameter in evaluating performance. As shown, the Si/PQ structure560 had a fill-factor of 17%, but the Si/organic monolayer structure 570had an increased fill-factor of 65%. The Si/organic monolayer structurealso demonstrated a substantial increase of short circuit density (morethan 2.5×).

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A heterojunction in a photovoltaic device,comprising: an inorganic substrate having a surface; an organicmonolayer on the surface of the inorganic substrate, the inorganicmonolayer having the following formula:˜X—Y, wherein X is an oxygen or a sulfur; Y is an alkyl chain, analkenyl chain, or an alkynyl chain; and X covalently bonds to thesurface of the inorganic substrate by a covalent bond; an organicsemiconductor material on a surface of the organic monolayer, theorganic semiconductor material being substantially free of doping; and aconductive electrode disposed onto at least on a portion of the organicsemiconductor material.
 2. The heterojunction of claim 1, wherein theinorganic substrate comprises silicon.
 3. The heterojunction of claim 1,the inorganic substrate comprises an n-type semiconductor.
 4. Theheterojunction of claim 3, wherein the conductive electrode comprises ahigh work-function material.
 5. The heterojunction of claim 1, whereinthe organic semiconductor has electron blocking properties.
 6. Theheterojunction of claim 1, wherein the organic semiconductor has holetransport properties.
 7. The heterojunction of claim 1, wherein Y hasabout 4 to about 21 carbons.
 8. The heterojunction of claim 1, whereinthe organic monolayer has a thickness in a range from about 0.5nanometers (nm) to about 5 nm.
 9. The heterojunction of claim 1, whereinthe inorganic substrate comprises a p-type semiconductor.
 10. Theheterojunction of claim 9, wherein the organic semiconductor has holeblocking properties, electron transport properties, or combinationthereof.
 11. A heterojunction in a photovoltaic device, comprising: aninorganic substrate having a surface; an organic monolayer on thesurface of the inorganic substrate, the inorganic monolayer having thefollowing formula:˜X—Y—Z, wherein X is an oxygen or a sulfur; Y is an alkyl chain, analkenyl chain, or an alkynyl chain; Z is a methyl group or an epoxygroup; and X covalently bonds to the surface of the inorganic substrateby a silicon-oxygen bond or a silicon-sulfur bond; an organicsemiconductor material on a surface of the organic monolayer, theorganic semiconductor material being substantially free of doping; and aconductive electrode disposed onto at least on a portion of the organicsemiconductor material.
 12. The heterojunction of claim 11, wherein theY is an alkenyl chain comprising all cis double bonds.
 13. Theheterojunction of claim 11, wherein the Y is an alkenyl chain comprisingall trans double bonds.
 14. The heterojunction of claim 11, wherein Y issubstantially free of branching.
 15. The heterojunction of claim 11,wherein the organic semiconductor has electron blocking properties. 16.The heterojunction of claim 11, wherein the organic semiconductor hashole transport properties.
 17. The heterojunction of claim 11, whereinthe organic monolayer has a thickness in a range from about 0.5nanometers (nm) to about 5 nm.
 18. The heterojunction of claim 11,wherein the inorganic substrate comprises a p-type semiconductor. 19.The heterojunction of claim 18, wherein the organic semiconductor hashole blocking properties, electron transport properties, or acombination thereof.
 20. The heterojunction of claim 11, wherein theconductive electrode comprises a high work-function material.